Raman Spectrum of Long-Wavelength Phonons in Tetragonal Barium Titanate

Polarized Raman spectra of tetragonal BaTiO₃ have been obtained at room temperature with He-Ne laser excitation. Using several different crystallographic directions for the phonon wave vector q, most of the vibrational modes for q{~}0 have been classified using Raman data alone. Three pairs of longitudinal-transverse A₁ modes have been observed with frequencies which semiquantitatively satisfy the Lyddane-Sachs-Teller relationship. A low-frequency transverse E mode has been observed at 10 cm^-1 and assigned to the ferroelectric soft-mode system. The soft-mode result has been discussed in terms of the effect of damping on the dispersion relation. These results are not in good agreement with those derived previously by multiparameter fits to the infrared reflectivity data.